The present invention relates to a method for manufacturing a rare-earth bonded magnet, and a rare-earth bonded magnet manufactured according to the method.
In general, a rare-earth bonded magnet is manufactured by molding a mixture or a kneaded mixture (compound) of a rare-earth magnetic powder and a binder resin (organic binder) into a desired magnet shape. For molding, compaction molding, injection molding or extrusion molding is employed.
In compaction molding, such a compound is placed into a mold and press-molded to obtain a green compact, and the compact is then heated to harden a thermosetting resin contained as a binder resin in the compound, thus manufacturing a bonded magnet. Since compaction molding is applicable to a composition including a smaller amount of the binder resin than that for other molding methods, the resin content in the obtained magnet can be reduced, and therefore, magnetic properties of the obtained magnet can be advantageously enhanced. In compaction molding, however, the variety of moldable magnet shapes is restricted, and productivity is low.
In injection molding, a compound is heat-melted so as to be sufficiently fluidized, and injected into a mold to be molded into a predetermined magnet shape. According to injection molding, versatility of shape can be high, and therefore, even irregular shaped magnets can be readily molded. In injection molding, however, since high fluidity is required of the melted compound, a large amount of binder resin must be added. The binder resin content in the obtained magnet therefore increases, which results in low magnetic properties.
In extrusion molding, a compound fed into an extruder is heat-melted, solidified by cooling in a die of the extruder, and extruded to obtain a long molded body. The molded body is then cut into magnet products having a desired length. According to extrusion molding, the advantages of both compaction molding and injection molding can be achieved. More specifically, the magnet shape can be relatively freely designed by appropriately selecting a die, namely, thin magnets and long magnets can be readily manufactured. Further, since such a high fluidity as is required of the melted compound in injection molding is not necessarily required, the amount of binder resin added to the compound can be smaller than that in injection molding, and therefore, the obtained magnet can exhibit enhanced magnetic properties.
Hitherto, screw extruders are used for extrusion molding. Such a screw extruder has a screw disposed in a heated cylinder, and raw material is forwarded while being kneaded by the rotation of the screw. Although such a screw extruder can extrude a compound continuously and quickly its generatable extruding pressure is relatively low (for example, approximately 200 to 500 kg/cm2). Due to this, in order to cope with such a low extruding pressure, the viscosity of the heat-melted compound in the extruder should be to some extent adjusted to a low level.
As a measure for reducing the compound viscosity, for example, the material temperature (die temperature) may be raised. This measure may, however, be restricted from matter concerning the composition, properties and the like of the binder resin, and thermostability and oxidation resistance of the magnetic powder.
Further, although the viscosity of heat-melted compound can be reduced in proportion to the content of the binder resin in the compound, magnetic properties of the obtained magnet will be lowered when the content of the binder resin is increased, as described above. As a result, the advantages of extrusion molding cannot be sufficiently exhibited.
Moreover, in such extrusion molding, since the raw material is horizontally extruded, the molded body may be deformed under the influence of gravity in the cross-sectional direction of the body (shearing stress).
In particular, when a round-rod or hollow cylindrical rare-earth bonded magnet is manufactured by such extrusion molding, the roundness of the magnet is reduced. Additionally, rare-earth bonded magnets having plate or thinner shapes, which generally have low strength, are readily deformed by the action of gravity during the manufacturing process, and in such cases, the obtained magnets exhibit lowered dimensional accuracy.
The object of the present invention is to provide a rare-earth bonded magnet having superior magnetic properties and dimensional precision and a method for manufacturing the same while taking advantage of the benefits of extrusion molding.
(1) The present invention provides a method for manufacturing a rare-earth bonded magnet, comprising extruding a rare-earth-bonded-magnet composition containing a rare-earth magnetic powder and a binder resin using an extruder, wherein the extruding direction by said extruder is substantially vertical.
(2) Preferably, said extruder is a ram extruder.
(3) Further, the present invention provides a method for manufacturing a rare-earth bonded magnet, comprising extruding a rare-earth-bonded-magnet composition containing a rare-earth magnetic powder, a binder resin and an antioxidant using an extruder, wherein the extruding direction by said extruder is substantially vertical.
(4) Preferably, said extruder is a ram extruder.
(5) Preferably, the total content of said binder resin and said antioxidant in said rare-earth-bonded-magnet composition is 10.0 to 22.4 vol %.
(6) Preferably, the content of said antioxidant in said rare-earth-bonded-magnet composition is 1.0 to 12.0 vol %.
(7) The content of said rare-earth magnetic powder in said rare-earth-bonded-magnet composition is 77.6 to 90.0 vol %.
(8) Moreover,the present invention provides a method for manufacturing a rare-earth bonded magnet containing a rare-earth magnetic powder and a binder resin, comprising:
a step of mixing a rare-earth magnetic powder and a binder resin to obtain a rare-earth-bonded-magnet composition;
an extrusion-molding step-in which said rare-earth-bonded-magnet composition is substantially vertically extruded using an upright extruder to obtain a long molded body; and
a step of cutting said extrusion-molded long body, wherein, in said extrusion-molding step, said binder resin which has been melted or softened is solidified in the outlet portion of a die.
(9) Furthermore, the present invention provides a method for manufacturing a rare-earth bonded magnet containing a rare-earth magnetic powder and a binder resin, comprising:
a step of mixing a rare-earth magnetic powder and a binder resin;
a step of kneading the thus obtained mixture at a temperature equal or higher than the thermal deformation temperature or softening temperature of said binder resin to obtain a rare-earth-bonded-magnet composition;
an extrusion-molding step in which said rare-earth-bonded-magnet composition is substantially vertically extruded using an upright extruder to obtain a long molded body; and
a step of cutting said extrusion-molded long body, wherein, in said extrusion-molding step, said binder resin which has been melted or softened is solidified in the outlet portion of a die.
(10) Preferably, said rare-earth-bonded-magnet composition comprises pellets or granules of the kneaded mixture.
(11) Preferably, said extruder is a ram extruder.
(12) Preferably, said rare-earth magnetic powder contains, as the main ingredients, rare-earth elements principally including Sm, and transition metals principally including Co.
(13) Preferably, said rare-earth magnetic powder contains, as the main ingredients, R (at least one element selected from rare-earth elements including Y), transition metals principally including Fe, and B.
(14) Preferably, said rare-earth magnetic powder contains, as the main ingredients, rare-earth elements principally including Sm, transition metals principally including Fe, and interstitial elements principally including N.
(15) Preferably, said rare-earth magnetic powder is a mixture comprising at least two rare-earth magnetic powders selected from those described in the above paragraphs (12), (13) and (14).
(16) Preferably, the extruding direction in said extrusion-molding step is downward-vertical.
(17) The present invention also provides a rare-earth bonded magnet characterized by being manufactured according to any one of the methods described in the above paragraphs (1) to (16).
(18) Preferably, said rare-earth bonded magnet has avoid ratio of 2 vol % or less.
(19) Preferably, said rare-earth bonded magnet has a round-rod shape or a hollow cylindrical shape, and a periphery Roundness of 5/100 mm or less [wherein, Roundness=(Maximum Outer Diameterxe2x88x92Minimum Outer Diameter)xc3x971/2].